Magnetic bearing apparatus

Abstract: In a magnetic bearing apparatus, a rotating member (1) having an axis of rotation along the horizontal direction is supported by a pair of radial magnetic bearings (4,5) each including a plurality of electromagnets (4a,4b,5a,5b) in the radial direction with clearances with the rotating member (1) in two portions in the axial direction. The apparatus comprises a pair of position detecting sensors (31,32) for detecting the displacement in the radial direction of the rotating member (1) in the two positions in the axial direction of the rotating member (1) and a control device (6) for controlling magnetic attraction forces (F1,F2) produced by the electromagnets (4a,4b,5a,5b) on the basis of output signals from the respective sensors (31,32). The control device (6) determines the magnetic attraction force (F1,F2) produced by each of the radial magnetic bearings (4,5) on the basis of the output signal from each of the sensors (31,32), a value of a control current supplied to the electromagnets (4a,4b,5a,5b) in the radial magnetic bearing (4,5), and an attraction force coefficient of the electromagnets in the radial magnetic bearing (4,5), to determine the position of the center of gravity of the rotating member on the basis of comparison of the calculated magnetic attraction forces (F1,F2).

Description: The invention relates to a magnetic bearing apparatus of the type as specified in the preamble of claim 1. Such an apparatus is known from JP-A-2 097 714. In particular, the invention relates to a magnetic bearing apparatus for raising a spindle of a machine tool and controlling the position thereof. There has been conventionally a magnetic bearing apparatus so adapted as to support a rotating member, for example, a rotor of a spindle of a machine tool by magnetic forces of two magnetic bearings disposed in two positions in the axial direction with clearances between the magnetic bearings and the rotor, detect the position of the rotor by a position sensor, and control a current supplied to electromagnets in the two magnetic bearings on the basis of a detection signal of the position sensor, thereby to hold the rotor in a reference position (a target position). Furthermore, in the above described control, the translation motion and the tilt motion concerning the center of gravity of the rotor are separately controlled. In the above described magnetic bearing apparatus, however, when a tool such as a grindstone or a cutter mounted on an end of the rotor is replaced on the way, the quantity of state such as the position of the center of gravity of the rotor is changed, so that position control becomes unstable, for example. Therefore, there is provided a control device so adapted as to input the weight of a tool by inputting means when the tool is replaced to determine, for example, the position of the center of gravity (see Japanese Patent Laid-Open Gazette No. 97714/1990 = JP-A-2 097 714). In the above described control device, however, the weight of the tool must be previously grasped, so that it takes a lot of time and labor to replace the tool. An object of the present invention is to provide a magnetic bearing apparatus which can automatically determine new position of the center of gravity even if an addition heavy product is mounted on a rotating member and can be suitably used particularly when the translation motion and the tilt motion concerning the center of gravity of the rotating member are separately controlled. The above object is solved by an apparatus as defined in claim 1. Further developments are covered by the subclaims. Thus in order to attain the above described object, the present invention provides a magnetic bearing apparatus comprising first and second radial magnetic bearings each including a plurality of electromagnets for exerting magnetic attraction forces produced by the electromagnets on a rotating member having an axis of rotation along the horizontal direction in two positions spaced apart from each other in the axial direction of the rotating member and respectively supporting the rotating member in the radial direction with clearances with the rotating member, first and second position detecting means for respectively detecting the positions in the radial direction of direction rotating member in the two positions in the axial direction of the rotating member, a control device for adjusting the magnetic attraction forces respectively produced by the electromagnets in the first and second radial magnetic bearings on the basis of output signals of the first and second position detecting means to hold the rotating member in a predetermined position in the radial direction, and means to adjust coefficients in the control device to take account of the position of the center of gravity, the means to adjust coefficients in the control device comprising a magnetic attraction force determining means for determining the magnetic attraction force produced by each of the radial magnetic bearings on the basis of the output signal of each of the position detecting means, a value of a control current supplied to the electromagnets in the radial magnetic bearing, and an attraction force coefficient of the electromagnets in the radial magnetic bearing, and a center-of-gravity position determining means for determining the position of the center of gravity of the rotating member on the basis of comparison of the determined magnetic attraction forces produced by both the radial magnetic bearings, whereby the output of the center-of-gravity position determining means is supplied to the control device. In accordance with the above described invention, the magnetic attraction force determining means determine the magnetic attraction force produced by each of the radial magnetic bearings on the basis of the output signal of each of the position detecting means, the value of the control current supplied to the electromagnets in the radial magnetic bearing and the attraction force coefficient of the electromagnets in the radial magnetic bearing, and the center-of-gravity position determining means determine the position of the center of gravity on the basis of the comparison of the determined magnetic attraction forces produced by both the radial magnetic bearings. Particularly, even when an addition product having an unknown weight is mounted on the rotating member, the position of the center of gravity is determined on the basis of the magnetic attraction force produced by each of the radial magnetic bearings after mounting the addition product, thereby making it possible to find an accurate position of the center of gravity. Consequently, the present invention is suitable for the application to an apparatus for supporting a spindle of a machine tool. In addition, the present invention is suitable for the application to a system of separately controlling the translation motion and the tilt motion concerning the center of gravity of the rotating member, that is, a separate control system. The reason for this is that in the separate control system, the position of the center of gravity of the rotating member must be determined in advance. Furthermore, the present invention is characterized, in a preferred mode, in that the output signal of each of the position detecting means is corrected by adding a correction signal so set that the result of the operation executed by the magnetic attraction force determining means becomes zero at the unloading time. In this case, a high-precision operation of the magnetic attraction forces can be executed, thereby making it possible to find the position of the center of gravity of the rotating member with high precision and consequently, to control the position of the rotating member with high precision. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing the construction of a magnetic bearing apparatus according to one embodiment of the present invention; Fig. 2 is a schematic diagram showing the positional relationship among a rotating member, electromagnets in magnetic bearings and position sensors in the magnetic bearing apparatus; Fig. 3 is a block diagram showing an X-axis direction control device; Fig. 4 is a block diagram showing a magnetic attraction force determining means; and Fig. 5 is a block diagram showing a modified example of a magnetic attraction force determining means. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the accompanying drawings showing embodiments. Fig. 1 shows a principal part of a magnetic bearing apparatus of a five-degrees-of-freedom control type and its control device as one embodiment of the present invention. In the following description, "upper", "lower" and "left" and "right" refer to the arrangement in Fig. 1. The magnetic bearing apparatus comprises one axial magnetic bearing (not shown) and two left and right radial magnetic bearings 2 and 3 for supporting a rotating member 1 with clearances with the rotating member 1. The axial magnetic bearing supports the rotating member 1 in the Z-axis direction (in the horizontal direction, that is, the right and left direction in Fig. 1). The radial magnetic bearings 2 and 3 support the rotating member 1 in the radial direction. Each of the radial magnetic bearings 2 and 3 comprises a portion for supporting the rotating member 1 in the X-axis direction orthogonal to the Z-axis (in the vertical direction, that is, the up and down direction in Fig. 1) (an X-axis direction magnetic bearing) and a portion for supporting the rotating member 1 in the Y-axis direction orthogonal to the Z-axis and the X-axis (the direction orthogonal to the paper of Fig. 1) (a Y-axis direction magnetic bearing). The X-axis direction magnetic bearings 4 and 5 are illustrated in Fig. 1. The radial magnetic bearings 2 and 3 comprise a portion for controlling the X-axis direction magnetic bearings 4 and 5 (an X-axis direction control device) and a portion for controlling the Y-axis direction magnetic bearings. The X-axis direction control device 6 is illustrated in Fig. 1. Furthermore, position sensors 31 and 32 for detecting the displacement of the rotating member 1 are respectively disposed in the vicinities of the radial magnetic bearings 2 and 3. The position sensors 31 and 32 respectively comprise portions for detecting the position of the rotating member 1 in the X-axis direction (position sensors 7a and 7b and 8a and 8b) and portions for detecting the position thereof in the Y-axis direction (not shown). The left radial magnetic bearing 2 shall be referred to as a first radial magnetic bearing, and the right radial magnetic bearing 3 shall be referred to as a second radial magnetic bearing. In addition, the left X-axis direction magnetic bearing 4 shall be referred to as a first magnetic bearing, and the right X-axis direction magnetic bearing 5 shall be referred to as a second magnetic bearing. The X-axis direction magnetic bearings 4 and 5 and the X-axis direction control device 6 shown in Fig. 1 separately control the translation motion and the tilt motion of the rotating member 1 on a X - Z plane. The translation motion is motion in the X-axis direction of the center of gravity of the rotating member 1, and the tilt motion is rotary motion of the rotating member 1 around an axis passing through the center of gravity of the rotating member 1 and parallel to the Y-axis. The present embodiment is characterized in that when an unknown addition product is mounted on the rotating member 1 to change the position of the center of gravity, a center-of-gravity position determining means 50 determines the position of the center of gravity on the basis of magnetic attraction forces F1 and F2 respectively determined by a pair of magnetic attraction force determining means 40A and 40B to alter coefficients concerning the position of the center of gravity for operations executed by the X-axis direction control device 6. The details of the operations executed by the magnetic attraction force determining means 40A and 40B and the center-of-gravity position determining means 50 will be described later. The X-axis direction magnetic bearings 4 and 5 respectively comprise a pair of electromagnets 4a and 4b and a pair of electromagnets 5a and 5b so disposed that the rotating member 1 is interposed therebetween on both its sides in the X-axis direction. The electromagnets 4a and 4b of the first magnetic bearing 4 shall be referred to as first electromagnets, and the electromagnets 5a and 5b of the second magnetic bearing 5 shall be referred to as second electromagnets. The above described pairs of position sensors 7a and 7b and 8a and 8b are respectively disposed in the vicinities of the electromagnets 4a and 4b and 5a and 5b of the X-axis direction magnetic bearings 4 and 5 so that the rotating member 1 is interposed therebetween on both its sides in the X-axis direction. The position sensors 7a and 7b disposed in the vicinity of the first magnetic bearing 4 shall be referred to as first position sensors, and the position sensors 8a and 8b disposed in the vicinity of the second magnetic bearing 5 shall be referred to as second position sensors. Outputs of the pair of first position sensors 7a and 7b are input to a first subtracter 10, thereby to determine displacement in the X-axis direction X1 on the left side of the rotating member 1. Outputs of the pair of second position sensors 8a and 8b are input to a second subtracter 11, thereby to determine displacement in the X-axis direction X2 on the right side of the rotating member 1. The first position sensors 7a and 7b and the first subtracter 10 constitute first position detecting means for detecting the position in the X-axis direction on the left side of the rotating member 1. The second position sensors 8a and 8b and the subtracter 11 constitute second position detecting means for detecting the position in the X-axis direction on the right side of the rotating member 1. A driving signal C1 of the first electromagnets 4a and 4b which is a first output of the control device 6 is input to a first power amplifier 12 constituting electromagnet driving means for driving the upper left electromagnet 4a, and is input to a second power amplifier 14 constituting electromagnet driving means for driving the lower left magnet 4b through an inverter 13. A constant stationary current is caused to flow through coils of the first electromagnets 4a and 4b when the driving signal C1 is zero, and the value of the current flowing through the coils is controlled by the driving signal C1. A driving signal C2 of the second electromagnets 5a and 5b which is a second output of the control device 6 is input to a third power amplifier 15 constituting electromagnet driving means for driving the upper right electromagnet 5a, and is input to a fourth power amplifier 17 constituting electromagnet driving means for driving the lower right electromagnet 5b through an inverter 16. A constant stationary current is also caused to flow through coils of the second electromagnets 5a and 5b when the driving signal C2 is zero, and the value of the current flowing through the coils is controlled by the driving signal C2. The values of the currents flowing through the coils of the first and second electromagnets 4a, 4b, 5a and 5b are thus controlled, whereby the translation motion and the tilt motion of the rotating member 1 are separately controlled. Fig. 2 shows the positional relationship among the rotating member 1, the electromagnets 4a, 4b, 5a and 5b, and the position sensors 7a, 7b, 8a and 8b. In Fig. 2, the center of gravity of the rotating member 1 is represented by G . The distance in the axial direction from the center of gravity G to the first electromagnets 4a and 4b is taken as M1, the distance in the same direction from the center of gravity G to the second electromagnets 5a and 5b is taken as M2, the distance in the same direction from the center of gravity G to the first position sensors 7a and 7b is taken as S1, and the distance in the same direction from the center of gravity G to the second position sensors 8a and 8b is taken as S2. In addition, it is assumed that M1 + M2 = MM (constant), and S1 + S2 = SS (constant). In addition, the amount of displacement in the X-axis direction of the center of gravity G is represented by XG, and the amount of tilt around the center of gravity G of the rotating member 1 on an X - Z plane, that is, the amount of tilt of the rotating member 1 around an axis parallel to the Y-axis passing through the center of gravity G is represented by Y. Fig. 3 shows one example of the control device 6. In Fig. 3, an output X1 of the first subtracter 10 is multiplied by [S2 / (S1 + S2)] in a first amplifier 20 and the product of X1 and [S2 / (S1 + S2)] is input to a first adder 21, an output X2 of the second subtracter 11 is multiplied by [S1 / (S1 + S2)] in a second amplifier 22 and the product of X2 and [S1 / (S1 + S2)] is input to a first adder 21. The respective products are added to each other in the first adder 21, thereby to determine the amount of displacement XG. In the first and second amplifiers 20 and 22, the outputs X1 and X2 are respectively multiplied by coefficients based on the distances S1 and S2 from the center of gravity G to the position sensors 7a and 7b and the position sensors 8a and 8b, and the respective products of X1 and X2 and the coefficients are added to each other. Even when the distances are not equal to each other, therefore, the amount of displacement XG can be always accurately determined. The first and second amplifiers 20 and 22 as well as the first adder 21 constitute displacement amount determining means for determining the amount of displacement XG of the rotating member 1 from the outputs of the first and second subtracters 10 and 11. The outputs of the first and second subtracters 10 and 11 are also input to two input terminals of a third subtracter 23, whereby the amount of tilt Y of the rotating member 1 is determined. The third subtracter 23 constitutes tilt amount determining means for determining the amount of tilt Y of the rotating member 1 from the outputs of the first and second subtracters 10 and 11. The output XG of the first adder 21 is input to a PID control circuit for translation motion 24 constituting translation motion control means, and such a translation motion control signal D1 that the amount of displacement XG becomes zero is output from the circuit 24. The output Y of the third subtracter 23 is input to a PID control circuit for tilt motion 25 constituting tilt motion control means, and such a tilt motion control signal D2 that the amount of tilt Y becomes zero is output from the circuit 25. The outputs D1 and D2 of the two control circuits 24 and 25 are input to input terminals of a second adder 26, and a driving signal C1 of the first electromagnets 4a and 4b is output from the adder 26. The translation motion control signal D1 is input to a fourth subtracter 28 through a third amplifier 27, and the tilt motion control signal D2 is input to a fourth subtracter 28 through a fourth amplifier 29. A first coefficient A1 times the translation motion control signal D1 is output from the third amplifier 27, and a second coefficient A2 times the tilt motion control signal D2 is output from the fourth amplifier 29. In the fourth subtracter 28, A2 times the tilt motion control signal D2 is subtracted from A1 times the translation motion control signal D1, so that a driving signal C2 of the second electromagnets 5a and 5b is output. The first signal A1 and the second signal A2 are represented as follows: A1 = (K1 / K2) · (I01/I02) · (M1 / M2) A2 = (K1 / K2) · (I01 / I02) In the equations, K1 is an attraction force coefficient of the first electromagnets 4a and 4b, K2 is an attraction force coefficient of second electromagnets 5a and 5b, I01 is a value of a stationary current supplied to the first electromagnets 4a and 4b, and I02 is a value of a stationary current supplied to the second electromagnets 5a and 5b. In order to control the translation motion and the tilt motion in a completely separate manner in the above described magnetic bearing apparatus, it is necessary that no tilt motion is newly brought about by attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the translation motion control signal D1, and no translation motion is newly brought about by attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the tilt motion control signal D2. The conditions under which no tilt motion is newly brought about by the attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the translation motion control signal D1 are that no moment for causing tilt motion is generated by the attraction force F1 of the first electromagnets 4a and 4b proportional to a control current value IC1 which is produced by the translation motion control signal D1 and the attraction force F2 of the second electromagnets 5a and 5b proportional to a control current value IC2 which is produced by the tilt motion control signal D2. The relationship between IC1 and IC2 satisfying the conditions is as follows: IC2 / IC1 = (K1 / K2) · (I01 / I02) · (M1 / M2) A right side in this equation is equal to the first coefficient A1. In the above described magnetic bearing apparatus, the translation motion control signal D1 is directly input to the second adder 26 for outputting the driving signal C1 of the first electromagnets 4a and 4b, and A1 times the translation motion control signal D1 is input to the fourth subtracter 28 for outputting the driving signal C2 of the second electromagnets 5a and 5b, so that IC1 and IC2 satisfy the conditions of the foregoing equation. Consequently, no tilt motion is newly brought about by the attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the translation motion control signal D1. The conditions under which no translation motion is newly brought about by the attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the tilt motion control signal D2 are that the attraction force F1 of the first electromagnets 4a and 4b proportional to the control current value IC1 which is produced by the tilt motion control signal D2 and the attraction force F2 of the second electromagnets 5a and 5b proportional to the control current value IC2 which is produced by the tilt motion control signal D2 are in balance. The relationship between IC1 and IC2 satisfying the conditions is as follows: IC2 / IC1 = (K1 / K2) · (I01 / I02) A right side in this equation is equal to the second coefficient A2. In the above described magnetic bearing apparatus, the tilt motion control signal D2 is directly input to the second adder 26 for outputting the driving signal C1 of the first electromagnets 4a and 4b, and A2 times the tilt motion control signal D2 is input to the fourth subtracter 28 for outputting the driving signal C2 of the second electromagnets 5a and 5b, so that IC1 and IC2 satisfy the conditions of the foregoing equation. Consequently, no translation motion is newly brought about by the attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the tilt motion control signal D2. In the above described magnetic bearing apparatus, therefore, no tilt motion is newly brought about by the attraction forces of the first and second magnets 4a, 4b, 5a and 5b which are produced by the translation motion control signal D1, and no translation motion is newly brought about by the attraction forces of the first and second electromagnets 4a, 4b, 5a and 5b which are produced by the tilt motion control signal D2, thereby making it possible to control the translation motion and the tilt motion in a completely separate manner. Description is now made of operations executed by the magnetic attraction force determining means 40A and 40B and the center-of-gravity position determining means 50 in a case where the tool is replaced, for example. A magnetic attraction force F exerted on an object by electromagnets is generally represented by F = K (I / X) 2 using a current I flowing through the electromagnets, the distance X between the electromagnets and the object and an attraction force coefficient K of the electromagnets. Therefore, the magnetic attraction force determining means 40A and 40B respectively determine a magnetic attraction force F1 of the first electromagnets 4a and 4b and a magnetic attraction force F2 of the electromagnets 5a and 5b by the following operation expressions: F1 = K1 [IX1a / (X0 + X1)] 2 - K1 [IX1b / (X0 - X1)] 2 F2 = K2 [IX2a / (X0 + X2)] 2 - K2 [IX2b / (X0 - X2)] 2 In the expressions, IX1a is a control current value of the first electromagnet 4a, IX1b is a control current value of the first electromagnet 4b, and X1 is the amount of displacement of the rotating member 1 toward the electromagnet 4b from the center of the electromagnets 4a and 4b. In addition, IX2a is a control current value of the second electromagnet 5a, IX2b is a control current value of the second electromagnet 5b, and X2 is the amount of displacement of the rotating member 1 toward the electromagnet 5b from the center of the electromagnets 5a and 5b. Further, X0 is a set air gap value between the rotating member 1 and each of the electromagnets 4a, 4b, 5a and 5b. Fig. 4 is a block diagram showing the magnetic attraction force determining means 40A. Referring to Figs. 1 to 4, the control current value IX1a of the first electromagnet 4a, the control current value IX1b of the first electromagnet 4b, the amount of displacement X1 of the rotating member 1 toward the electromagnet 4b from the center of the electromagnets 4a and 4b, and the signal X0 corresponding to the set air gap value are inputted to the magnetic attraction force determining means 40A. Referring to Fig. 4, signals having a value of (X0 + X1) and a value of (X0 - X1) obtained by addition and subtraction in an adder 41 and a subtracter 42 are respectively signals corresponding to gaps between the electromagnets and the rotating member 1. Dividers 43 and 44 respectively receive the signals IX1a and IX1b and the signals (X0 + X1) and (X0 - X1) to execute operations IX1a / (X0 + X1) and IX1b / (X0 - X1). Reference numerals 45 and 46 denote square multipliers for executing a square operation, which respectively receive signals from the divider 43 and 44 to execute operations, thereby to obtain signals of [IX1a / (X0 + X1)] 2 and [IX1b / (X0 - X1)] 2 . The signal of [IX1b / (X0 - X1)] 2 is subtracted from the signal of [IX1a / (X0 + X1)] 2 by a subtracter 47, and the result of the subtraction is multiplied by K1 by a coefficient multiplier 48, thereby to obtain a signal F1 having a value of K1 [IX1a / (X0 + X1)] 2 - K1 [IX1b / (X0 - X1)] 2 . In addition, the control current value IX2a of the second electromagnet 4a, the control current value IX2b of the second electromagnet 5b, the amount of displacement X2 of the rotating member 1 toward the electromagnet 5b from the center of the electromagnets 5a and 5b, and the signal X0 corresponding to the set air gap value are input to the magnetic attraction force determining means 40B, thereby to obtain a signal F2 in the same manner as that in the case of the magnetic attraction force determining means 40A. Description is now made of operations executed by the center-of-gravity position determining means 50 for finding a new position of the center of gravity. The following equation holds from a balance between moments generated by forces around the center of gravity G : M1 / M2 = F2 / F1 On the other hand, the following equation holds: M1 + M2 = MM (constant) Accordingly, the respective distances M1 and M2 from the center of gravity G to the first and second electromagnets 4a and 4b and 5a and 5b are found from both the following equations (9) and (10) on the basis of the equations (7) and (8): M1 = MM / [1 + (F1 / F2)] M2 = MM / [1 + (F2 / F1)] Furthermore, the following equations hold: S1 = M1 + MS S2 = M2 + MS From the equations (9) and (12), the respective distances S1 and S2 from the center of gravity G to the first and second position sensors 7a and 7b and 8a and 8b are as follows: S1 = MM / [1 + (F1 / F2)] + MS S2 = MM / [1 + (F2 / F1)] + MS When the position of the center of gravity G is changed by replacing the tool, for example, the coefficient A1 in the third amplifier 27 is altered on the basis of a new value of M1 / M2 obtained by the foregoing equation (3) which is determined by the center-of-gravity position determining means 50. Furthermore, the respective coefficients in the first amplifier 20 and the second amplifier 22 are altered on the basis of new values of S1 and S2. According to the present embodiment, even if an addition product having a unknown weight is mounted on the rotating member 1, it is possible to carry out control which is adapted to a new state only by automatically determining the position of the center of gravity G on the basis of comparison of the magnetic attraction forces F1 and F2 of the magnetic bearings 4 and 5 after the mounting and altering some of the coefficients on the basis of the determined position of the center of gravity G . Consequently, the present invention can be easily applied to a separate control system for separately controlling the translation motion and the tilt motion concerning the center of gravity G of the rotating member 1. Particularly, the present invention is suitable for the application to a rotor of a spindle of a machine tool. Fig. 5 shows another embodiment of the present invention. Referring to Fig. 5, the present embodiment differs from the embodiment shown in Fig. 1 in that there is provided an adder 49 for adding a correction signal Xmis for correcting misalignment to a signal X0 corresponding to a set air gap, and the signal X0 corresponding to the set air gap is replaced with a signal obtained by the addition (X0 + Xmis). The correction signal Xmis is previously set by volume control or the like so that the result of the determination of the magnetic attraction force F1 by the magnetic attraction force determining means 40A becomes zero at the unloading time. In this case, a high-precision determination of the magnetic attraction force can be executed, thereby making it possible to find the position of the center of gravity G of the rotating member 1 with high precision and consequently, to control the position of the rotating member 1 with high precision. Although the present invention has been described and illustrated in detail, it is clearly understood that the description is by way of illustration and example only and is not to be taken by way of limitation.

Claims: A magnetic bearing apparatus comprising: first and second radial magnetic bearings (4, 5) each including a plurality of electromagnets (4a, 4b, 5a, 5b) for exerting magnetic attraction forces (F1, F2) produced by the electromagnets on a rotating member (1) having an axis of rotation along the horizontal direction (z) in two positions spaced apart from each other in the axial direction of the rotating member (1) and respectively supporting the rotating member (1) in the radial direction with clearances with the rotating member (1); first and second position detecting means (31, 32) for respectively detecting the positions in the radial direction of the rotating member (1) in the two positions in the axial direction of the rotating member (1); a control device (6) for adjusting the magnetic attraction forces (F1, F2) respectively produced by the electromagnets (4a, 4b, 5a, 5b) in the first and second radial magnetic bearings (4, 5) on the basis of output signals (X1, X2) of the first and second position detecting means (31, 32) to hold the rotating member (1) in a predetermined position in the radial direction; and means (20, 22, 27) to adjust coefficients in the control device (6) to take account of the position of the center of gravity (G), characterized in that the means (20, 22, 27) to adjust coefficients in the control device (6) comprises: magnetic attraction force determining means (40A, 40B) for determining the magnetic attraction force (F1, F2) produced by each of the radial magnetic bearings (4, 5) on the basis of the output signal (X0, X1, X2) of each of the position detecting means (31, 32), a value (IX1a, IX1b, IX2a, IX2b) of a control current supplied to the electromagnets (4a, 4b, 5a, 5b) in the radial magnetic bearing (4, 5), and an attraction force coefficient (K1,K2) of the electromagnets (4a, 4b, 5a, 5b) in the radial magnetic bearing (4, 5), and center-of-gravity position determining means (50) for determining the position of the center of gravity (G) of the rotating member (1) on the basis of comparison of the determined magnetic attraction forces (F1, F2) produced by both the radial magnetic bearings (4, 5), whereby the output (S1, S2, M1/M2) of the center-of-gravity position determining means (40A, 40B) is supplied to the control device (6). The apparatus according to claim 1, wherein the output signal of each of the first and second position detecting means (31, 32) is corrected by adding a correction signal (Xmis) so set that the result of the determination exerted by the magnetic attraction force determining means (40A) becomes zero at the unloading time. The apparatus according to claim 1 or 2, further comprising: displacement amount determining means (20, 21, 22) for determining the amount of radial displacement (XG) of the determined position of the center of gravity (G) of the rotating member (1) on the basis of the output signals of the first and second position detecting means (31, 32); tilt amount determining means for determining the amount of tilt (Y) around the determined position of the center of gravity (G) of the rotating member (1) on the basis of the output signals of the first and second position detecting means (31, 32); translation motion control means (24) for outputting such a translation motion control signal that the amount of displacement (XG) of the determined position of the center of gravity (G) of the rotating member (1) becomes zero; tilt motion control means (25) for outputting such a tilt motion control signal that the amount of tilt (Y) around the determined position of the center of gravity (G) of the rotating member (1) becomes zero; and first and second electromagnet driving means (14, 17) for respectively driving the electromagnets (4a, 4b, 5a, 5b) in the first and second radial magnetic bearings (4, 5) on the basis of outputs of the translation motion control means (24) and the tilt motion control means (25).

Inventors: UEYAMA HIROCHIKA, UEYAMA, HIROCHIKA, UEYAMA, HIROCHIKA, C/O KOYO SEIKO CO., LTD.
Applicants: KOYO SEIKO CO, KOYO SEIKO CO., LTD.
Application year: 1994
Main classification: F16C39
IPC codes: F16C32, F16C39